1. Field of the Invention
The present invention relates to an inductor.
2. Description of the Background Art
In the background art, there has been known an electronic component, that is, a so-called inductor, in which conducting wires are wound around a magnetic substance to obtain a desired inductance value or conducting wires are wound around an air core (such as a nonmagnetic bobbin, or nothing to serve as a core) to obtain a desired inductance value.
FIGS. 4A and 4B show a configuration example of a background-art inductor. FIG. 4A is a top view, and FIG. 4B is a side view.
A background-art inductor 50 has a configuration, for example, in which windings are formed by winding conducting wires around a magnetic substance 55. The magnetic substance 55 has a donut-like shape in the example shown in FIGS. 4A and 4B. The magnetic substance 55 may be regarded as an example of a so-called “toroidal core”. In addition, in the configuration example shown in FIGS. 4A and 4B, windings are formed out of two conducting wires (illustrated as two conducting wires 53 and 54). This configuration will be described below.
First, when a high-frequency current flows in a conducting wire, the current generally flows only near the surface of the conducting wire due to skin effect so as to increase loss. Therefore, windings are formed by use of a plurality of parallel conducting wires to increase the surface area of the conducting wires. When, for example, the two conducting wires 53 and 54 are used as shown in FIGS. 4A and 4B, the conducting wires 53 and 54 are connected in parallel between leader lines 51 and 52 of the inductor and wound around the magnetic substance 55.
The leader lines 51 and 52 may be regarded as terminals shared between the two conducting wires 53 and 54 or may be regarded as terminals for connecting the inductor 50 to some circuit.
Any number of conducting wires may be used. The number of conducting wires is not limited to two as shown in the example, but may be three, four, five, . . . or the like. When the number of conducting wires is increased thus, the surface area is increased, the sectional area where a high-frequency current can flow is also increased, and the effect of suppressing the loss particularly in the case where a high-frequency current flows in the conducting wires is enhanced, in comparison with when the number of conducting wires is one.
Although an example in which the magnetic substance 55 is used as a core has been illustrated here, an air-core coil in which conducting wires are wound around a bobbin or the like for fixing the conducting wires without use of any magnetic substance may be used alternatively. Also in this case, a plurality of conducting wires may be used.
There has been known another background-art technique as disclosed in JP-A-62-7101.
The invention disclosed in JP-A-62-7101 relates to a common mode choke coil for coping with common mode noise. As shown in FIGS. 4 and 5 of JP-A-62-7101, a common mode choke coil is typically designed as follows. That is, a pair of windings 2 and 3 are provided so that magnetic fluxes generated in a magnetic core 1 in response to a round current (normal mode current) can be cancelled with each other. Thus, the common mode chock coil can serve as an inductor for common mode noise.
On the other hand, the aforementioned inductor shown in FIGS. 4A and 4B may be regarded as a configuration for coping with normal mode noise. In such a case, the inductor is called a normal mode choke coil.
In addition, the inductor shown in FIGS. 4A and 4B has a two-terminal configuration while the common mode choke coil in JP-A-62-7101 has a four-terminal configuration in which inductors are inserted into two wires of an AC line respectively.
The inductor (normal mode choke coil) shown in FIGS. 4A and 4B by way of example has a problem about parasitic capacitance as will be described below. A solution to the problem has been required.
Here, FIG. 5 shows an equivalent circuit of the background-art inductor 50 shown in FIGS. 4A and 4B.
The equivalent circuit has a configuration in which parasitic capacitances are connected in parallel to inductance L depending on the inductor 50, as illustrated in FIG. 5. Parasitic capacitances between wires of windings are generally known well. Parasitic capacitances a-1 to a-N illustrated in FIG. 5 correspond to these parasitic capacitances. As shown in FIG. 4A, the parasitic capacitances a-1 to a-N are electrostatic capacitances generated between adjacent wires in the windings. That is, each of the parasitic capacitances a-1 to a-N is a parasitic capacitance generated between adjacent conductors in each turn of conducting wires (53 and 54). Accordingly, N parasitic capacitances are generated in series in N turns of the windings. Therefore, the parasitic capacitances a-1 to a-N are connected in series in the equivalent circuit as illustrated in FIG. 5. The reference sign N designates the number of turns (winding number) of the windings. The parasitic capacitance is also called inter-wire capacitance, winding capacitance, or the like.
In the inductor configured as shown in FIGS. 4A and 4B, parasitic capacitance is also generated between the leader line 51 and the leader line 52. Parasitic capacitance b shown in FIG. 4A corresponds to the parasitic capacitance. As shown in FIG. 5, configuration is made so that the parasitic capacitance b is connected in parallel to the aforementioned configuration in which the parasitic capacitances a-1 to a-N are connected in series.
In the inductor configured as shown in FIGS. 4A and 4B, windings are formed to make a round of the donut-like magnetic substance. Thus, the distance between the leader line 51 and the leader line 52 is apt to be small enough to generate the parasitic capacitance b between the leader lines 51 and 52. As shown in the equivalent circuit of FIG. 5, the parasitic capacitance b is connected not in series with the parasitic capacitances a-1 to a-N but in parallel therewith, so as to give a great influence to total parasitic capacitance of the inductor. Thus, the total parasitic capacitance is increased conspicuously. This will be described below.
Here, description will be made on the assumption that the values of the parasitic capacitances a-1 to a-N and the parasitic capacitance b are all the same (=C) (that is, on the assumption that conducting wires are adjacent to each other with the same area and with the same distance). In this case, the total parasitic capacitance between the leader line 51 and the leader line 52 can be expressed by “C+C/N”.
When the parasitic capacitance b is absent, the total parasitic capacitance is expressed by “C/N”. Therefore, if the number N of turns is increased, the total parasitic capacitance will be a very small value. In fact, however, large parasitic capacitance is generated in the inductor due to the existence of the parasitic capacitance b as described above.
Even if the distance between adjacent conducting wires is not fixed but variable, the average distance will be fixed so that the total parasitic capacitance will remain unchanged. In addition, parasitic capacitance generated between wires not adjacent to each other takes a small value due to a long distance between the wires not adjacent to each other. Thus, such parasitic capacitance is negligible. Further, when adjacent conductors have the same potential, energy cannot be stored in parasitic capacitance generated between the adjacent conductors. Thus, such parasitic capacitance is negligible compared to the parasitic capacitance between the leader line 51 and the leader line 52.
When an inductor having such a large parasitic capacitance is used in a conversion circuit or the like, there arises the problem that the current for charging/discharging the parasitic capacitance increases and hence loss or high-frequency noise increases. The operation in the case where the inductor 50 having large parasitic capacitance is applied to a power factor correction circuit shown in FIG. 6 will be described by way of example.
The power factor correction circuit shown in FIG. 6 includes an AC power supply 61, a diode bridge 62, the inductor 50, a diode 66, a switching device 65 and a capacitor 67. The inductor 50 is an inductor configured as shown in FIGS. 4A and 4B. The inductor 50 is connected to the power factor correction circuit through leader lines 51 and 52 (terminals). As shown in FIG. 6, the leader line 51 is connected to the diode bridge 62 side, and the leader line 52 is connected to the diode 66 side. In the inductor 50, inductance L is obtained while parasitic capacitance 68 (the aforementioned total parasitic capacitance=C+C/N) arranged in parallel to the inductance L is generated.
When the switching device 65 is OFF and the diode 66 is ON in the power factor correction circuit shown in FIG. 6, a current flows in a path from the AC power supply 61 through the diode bridge 62, the inductor 50, the diode 66, the capacitor 67 and the diode bridge 62 back to the AC power supply 61. A difference between an AC power supply voltage and an output voltage is applied to the inductor 50.
Here, when the switching device 65 is turned ON, the path of the current changes to a path from the AC power supply 61 through the diode bridge 62, the inductor 50, the switching device 65 and the diode bridge 62 back to the AC power supply 61. Thus, the voltage of the inductor 50 changes to AC power supply voltage suddenly as soon as the switching device 65 is turned ON. On this occasion, the voltage of the parasitic capacitance 68 also changes suddenly. Therefore, when the switching device 65 is turned ON, a sharp spike-like current for charging the parasitic capacitance 68 flows in a path from the AC power supply 61 through the diode bridge 62, the parasitic capacitance 68, the switching device 65 and the diode bridge 62 back to the AC power supply 61. This current flows as soon as the switching device 65 is turned ON. Therefore, the current is repeated with a switching frequency to thereby increase the switching loss of the switching device 65.
In addition, when there is large parasitic capacitance in the inductor 50, high-frequency conducted noise generated due to switching in the switching device 65 or the diode 66 leaks to the power system side through the parasitic capacitance 68. To attenuate the noise, a noise filter (not shown here) must be enhanced. Thus, the apparatus is made larger in size and higher in cost. The power factor correction circuit has been described here by way of example. However, when an inductor with large parasitic capacitance is used even in any other circuit, the inductor causes similar problems (such as increase of loss, increase of conducted noise, etc.).
Such a problem is merely an example. Although such a problem is not limited to this example, existence of large parasitic capacitance in the inductor 50 is not desirable anyway. However, particularly in the case of a normal mode choke coil etc. in the configuration in which the distance between two leader lines (terminals) is short, the total parasitic capacitance increases.
In order to solve the aforementioned problem, for example, it can be considered that conducting wires 53 and 54 are wound not to make a round but to make half a round (180 degrees) or ¾ of a round (270 degrees) so as to increase the distance between the leader line 51 and the leader line 52, by way of example. In this case, however, there is formed a portion (dead space) in which no winding is formed on the magnetic substance 55. Thus, there arise a problem that a desired number of turns cannot be obtained, a problem that a winding cannot be thickened to a desired thickness, a problem that the conduction loss increases, etc.
The configuration of the inductor is not limited to the example shown in FIGS. 4A and 4B. The example shown in FIGS. 4A and 4B has a configuration in which conducting wires are wound around a magnetic substance. As another configuration of the inductor, a so-called “air-core coil” or the like may be used. As described above, the air-core coil has a configuration in which conducting wires are wound around a bobbin for fixing the conducting wires without use of any magnetic substance. It is not necessary to be limited to this configuration, but another configuration using no bobbin (no core around which conducting wires should be wound) may be used.